First of all you need images to show as wallpaper. You can easily get them from various wallpaper sites on the web. Make sure you store the files in a single directory – you can organize them into sub-directories if there are too many.
Install feh which is an application for viewing images. This is in /usr/ports/graphics .
There are other wallpaper setting applications which can be used:
Once installed, run fbsetbg (fluxbox set background) . Run it first with fbsetbg -i which makes sure that there is a suitable application to set the wallpaper. You can now set the background wallpaper by using the command
fbsetbg -f /path/to/graphic/file
Eg. fbsetbg -f /var/data/wallpaper.jpg
You can even specify wildcards and fbsetbg will set the first matching file as the wallpaper.
Given below is a sample full-blown fluxbox installation.